The invention relates to time-division switching networks.
Such switching networks are known and may be TST, TSST, STS etc . . . networks, that is to say incorporating at least one space-division multiplex stage between two time-division stages, or else at least one time-division stage between two space-division multiplex stages.
In the case of a TST or TSST switching network, each time-division stage consists of time-division switches each associated with a switching interface linked to time division multiplex (TDM) or space division multiplex (SDM) signal lines. One time-division switch and the switching interface associated with it form a switching unit.
The failure of a switching unit in the incoming stage or the outgoing stage renders the SDM or TDM signal lines linked to this switching unit unserviceable consequently making it impossible to handle traffic from or to subscribers served by the SDM or TDM lines.
Solutions have been proposed to avoid such a problem.
One known solution consists in connecting each incoming switching unit to the preceding and following incoming switching units. In case of failure, each incoming switching unit acts as a backup to the following switching unit in accordance with a cyclic transposition system (if the transposition is reversed, clearly it is the preceding incoming unit which acts as the backup to the faulty switching unit).
In another known solution, each time-division switch in the incoming stage forms part of a module which also incorporates its two concentrators in parallel linked to the module's incoming subscriber lines. The outlet of one of the concentrators is multiplex linked to the time-division switch and the outlet of the other concentrator is multiplex linked to the time-division switch of the following module. If one module fails, with the exception of the concentrator linked to the following module, the traffic which was normally handled by the faulty module is distributed step by step over the other modules.
These solutions have the disadvantage of transferring all the traffic of the faulty switching unit or module on to the adjacent switching unit; in the first known solution this involves dividing the space-division switch between the incoming and outgoing switching units into two and duplicating the speech memory of each incoming and outgoing switching unit; in the second solution it involves duplicating the line concentrators and considerably increasing the subscriber line concentration rate and therefore the blocking rate.
Another known solution consists in using an additional standby switching unit for a number of switching units, to handle the traffic of the faulty switching unit. This solution requires the additional switching unit to be linked to all the other switching units, involving a not insubstantial wiring cost and the need to monitor it since it too may well become faulty.
A method is also known in which an incoming stage switching unit is associated with an outgoing stage switching unit to form a terminal module; in the last solution quoted it is a standby terminal module which handles the traffic from a faulty terminal module; such a standby terminal module associated with a number of terminal modules forms a group and the switching network therefore contains several of these groups.